1. FIELD OF THE INVENTION
This invention relates to an improved apparatus for effecting a change in direction of a rotary drill bit relative to a tubing string to change the direction of the bore of a subterranean well.
2. DESCRIPTION OF THE PRIOR ART
The changing of the direction of drilling of the bore of a subterranean well is an expedient long practiced by well drillers. In many instances, the change in direction is to produce a straightening of the well due to the deflection of the rotary drill from the desired direction by a particular rock strata. In other instances, the change in direction is intentional in order to reach a formation that is laterally displaced from the existing position of the bore hole.
One of the most common expedients for changing the direction of drilling has been the insertion in the tubing, drilling or work string formed by interconnection tubing sections at a point above the fluid motor which drives the rotary drill bit, an apparatus which is called a "conventional bent sub". Such conventional bent subs are rigidly connected at one end to the tubing string and have their other connecting end angularly disposed relative to the axis of the tubing string to which they are connected, so that a fluid motor and supported drill bit are rigidly connected thereto. The axis of the drill bit will be angularly inclined relative to the axis of the well bore existing prior to insertion of the conventional bent sub.
Because the change in angle has heretofore been accomplished at a substantial distance above the rotary drill bit, a large degree of interference is created between the conventional bent sub, the rotary drill bit and the well bore wall as the tubing string is lowered into the hole to where drilling will begin. To effect a desired change in angle, which generally is on the order of a fraction of a degree, it was necessary to remove the motor and drill bit from the end of the work string and insert a particular conventional bent sub which had the desired angular deviation incorporated therein. This method requires the maintenance of an inventory of conventional bent subs having different deviation angles at the drilling site.
There is, therefore, a recognized need in the well drilling industry for an apparatus which will permit a selected, change in well bore drilling direction to be effected without the large degree of interference between the tubing string, drill bit and bore wall and also to avoid maintenance of a large inventory of conventional bent subs.